As to their origin, it may be supposed that they are a relic of a ring of matter left in suspension during the contraction of the globe of Saturn from a nebulous mass, just as the rings from which the various planets are supposed to have been formed were left off during the contraction of the main body of the original solar nebula. Other similar rings originally surrounding Saturn may have become satellites, but the matter composing the existing rings is so close to the planet that it falls within the critical distance known as "Roche's limit," within which, owing to the tidal effect of the planet's attraction, no body so large as a true satellite could exist, and accordingly in the process of formation of the Saturnian system this matter, instead of being aggregated into a single satellite, has remained spread out in the form of a ring, although its substance long ago passed from the vaporous and liquid to the solid form. We have spoken of the rings as being composed of meteorites, but perhaps their component particles may be so small as to answer more closely to the definition of dust. In these rings of dust, or meteorites, disturbances are produced by the attraction of the planet and that of the outer satellites, and it is yet a question whether they are a stable and permanent feature of Saturn, or will, in the course of time, be destroyed.[12] [Footnote 12: For further details about Saturn's rings, see The Tides, by G.H. Darwin, chap. xx.] It has been thought that the gauze ring is variable in brightness. This would tend to show that it is composed of bodies which have been drawn in toward the planet from the principal mass of the rings, and these bodies may end their career by falling upon the planet. This process, indefinitely continued, would result in the total disappearance of the rings--Saturn would finally swallow them, as the old god from whom the planet gets its name is fabled to have swallowed his children. Near the beginning of this chapter reference was made to the fact that Saturn's rings have been regarded as habitable bodies. That, of course, was before the discovery that they were not solid. Knowing what we now know about them, even Dr. Thomas Dick, the great Scotch popularizer of astronomy in the first half of the nineteenth century, would have been compelled to abandon his theory that Saturn's rings were crowded with inhabitants. At the rate of 280 to the square mile he reckoned that they could easily contain 8,078,102,266,080 people.